Add CUDA compiler support for CC 5.0

With CUDA 6.5 and later compute capability 5.0 devices are supported, so
we generate cubin and PTX for these too and remove PTX 3.5.
This change also removes explicit optimization for CC 2.1 where
sm_20 binary code runs equally fast as sm_21.

Change-Id: I5a277c235b873afb2d1b2b12b5db64b370f1bade